1. Field of the Invention
The present invention relates to Mach-Zehnder interferometer type optical modulators.
2. Description of the Related Art
Patent Document 1 (Japanese Unexamined Patent Application Publication No. 62-183406) describes a waveguide-type optical interferometer. This waveguide-type optical interferometer includes a substrate, two optical guides composed of glass or a plastic formed on the substrate, two optical couplers that connect the optical waveguides to each other at different positions, and phase shifters disposed in the optical waveguides between the optical couplers. Each phase shifter includes a heater disposed on the optical waveguide. The optical path length of the optical waveguide is changed by controlling the temperature of the optical waveguide by heating the heater of the phase shifter.
In recent years, optical modulators that modulate light in response to electric signals from outside have become one of the essential components in configuring optical fiber communication systems and optical information processing systems. In particular, a Mach-Zehnder interferometer type optical modulator that uses a waveguide-type optical interferometer described in Patent Document 1 enables high-speed modulation of 40 Gbps or higher. Since Mach-Zehnder interferometer type optical modulators have a low wavelength chirp under high-speed modulation, Mach-Zehnder interferometer type optical modulators can be used for future ultra high-speed, high-capacity optical communication systems. In particular, Mach-Zehnder interferometer type optical modulators composed of semiconductors are small in size, have low power consumption, and can be monolithically integrated with other semiconductor optical devices such as a laser diode through to achieve wider versatility.
A Mach-Zehnder interferometer type optical modulator has an optical waveguide structure constituted by an upper cladding layer, a lower cladding layer, and a core layer between these cladding layers. The core layer is composed of a material having a refractive index higher than those of the upper cladding layer and the lower cladding layer. In particular, an optical waveguide structure of a semiconductor optical device has a pin structure in which one of the upper and lower cladding layers is composed of an n-type semiconductor, the other of the upper and lower cladding layers is composed of a p-type semiconductor, and the core layer is composed of an undoped semiconductor.
FIGS. 17, 18A and 18B show a Mach-Zehnder interferometer type optical modulator 100 having a pin structure. FIG. 17 is a plan view of the Mach-Zehnder interferometer type optical modulator 100. FIG. 18A is a cross-sectional view taken along line XVIIIa-XVIIIa in FIG. 17. FIG. 18B is a cross-sectional view taken along line XVIIIb-XVIIIb in FIG. 17.
As shown in FIG. 17, the Mach-Zehnder interferometer type optical modulator 100 includes a phase shifting section 110, an input optical coupler 120, an output optical coupler 130, and six optical waveguides 140a, 140b, 140c, 150a, 150b, and 150c. These components are formed on an n-type semiconductor substrate 101 (refer to FIGS. 18A and 18B). Each of the waveguides 140b and 150b has one end connected to the input optical coupler 120 and the other end connected to the output optical coupler 130. The phase shifting section 110 is interposed between the input optical coupler 120 and the output optical coupler 130. Upper electrodes 111a and 111b are respectively disposed on the optical waveguides 140b and 150b in the phase shifting section 110.
The optical waveguides 140a and 150a each extend from one end 100a of the Mach-Zehnder interferometer type optical modulator 100 to the input optical coupler 120. The optical waveguides 140c and 150c each extend from output optical coupler 130 to the other end 100b of the Mach-Zehnder interferometer type optical modulator 100.
Referring now to FIGS. 18A and 18B, the Mach-Zehnder interferometer type optical modulator 100 includes an n-type lower cladding layer 103, core layers 104a and 104b, p-type upper cladding layers 105a and 105b, and p-type contact layers 106a and 106b. The core layer 104a is interposed between the n-type lower cladding layer 103 and the p-type upper cladding layer 105a. The core layer 104b is interposed between the n-type lower cladding layer 103 and the p-type upper cladding layer 105b. The p-type contact layers 106a and 106b are disposed on the p-type upper cladding layers 105a and 105b, respectively. The upper electrodes 111a and 111b are disposed on the p-type contact layers 106a and 106b, respectively. A cathode electrode 112 is formed on the back of the n-type semiconductor substrate 101.
A part of the n-type lower cladding layer 103, the core layer 104a, the p-type upper cladding layer 105a, and the p-type contact layer 106a form a mesa structure 107a. The mesa structure 107a constitutes the optical waveguide 140b. Similarly, another part of the n-type lower cladding layer 103, the core layer 104b, the p-type upper cladding layer 105b, and the p-type contact layer 106b form another mesa structure 107b. The mesa structure 107b constitutes the optical waveguide 150b. Side surfaces of the mesa structures 107a and 107b are buried by, for example, a polyimide resin 108.
According to the Mach-Zehnder interferometer type optical modulator 100, the refractive indices of the core layers 104a and 104b can be changed by applying a reverse bias voltage between the cathode electrode 112 and the upper electrodes 111a and 111b. As a result, the phase of the light guided in the core layers 104a and 104b can be shifted.